The invention relates to a process and associated apparatus for the catalytic dehydrogenation of a C.sub.2+ paraffinic hydrocarbon charge. It relates more particularly to the inhibition of freezing of the water contained in the effluent before the cooling and the separation of the effluent.
U.S. Pat. Nos. 3,536,775 and 3,663,641 describe a process for removing, by contact with water, the oxygen contained in a butadiene cut obtained by an oxidizing dehydrogenation of a butene cut. Patent EP-A-7750 describes a process for removing the water contained in nonolefinic light hydrocarbons such as natural gas, liquefied gases, gasolines or kerosene, by adding aqueous methanol. Under these conditions, a hydrocarbon liquid phase containing methanol which prevents the formation of hydrates is obtained. Moreover, the methanol is not recovered from the aqueous liquid phase.
Finally, U.S. Pat. No. 3,663,641 illustrates the technological background.
It is known that a number of industrial processes using low pressure catalytic reactions operate in a hydrogen environment in which the partial pressure of hydrogen is assured by a recycling of a hydrogen-rich gas contained in a reaction effluent and which has been separated from the hydrocarbons.
This is the case in particular of a catalytic dehydrogenation process of LPG's containing propane, butane and isobutane to produce mono-olefins which serve as an intermediate for the production of fuel with high octane number. In the case of the dehydrogenation of isobutane, the isobutene produced can react with methanol to produce methyl tert-butyl ether, an additive that can be used in gasolines.
The prior art is illustrated by U.S. Pat. Nos. 4,381,418 and 4,381,417. In such processes, the reaction is performed in a continually regenerated catalytic reactor operating at very low pressure (slightly higher than the atmospheric pressure or under vacuum) and at temperatures of 500.degree. C. to 600.degree. C.
The recycled hydrogen and the hydrogen produced assure sufficient partial pressure of the hydrogen to inhibit the formation of coke and thereby to maintain the stability of the catalyst. Thus, a satisfactory conversion to a higher range of temperatures reaching, for example, 600.degree. C., is achieved. Generally, the low pressure effluent delivered by the dehydrogenation reaction zone is cooled first of all by heat exchange with the gas charge then with water, at a suitable temperature before the vapor pressure of the effluent is effectively raised, in a conventional piece of compression equipment, at a higher pressure, which makes possible the separation of the hydrogen and the hydrocarbon compounds of the effluent. These conventional exchangers and other air coolers increase the counterpressure of the reaction system, which negatively affects the conversion rate. To eliminate these drawbacks, it has been attempted to use heat exchangers with slight pressure drop and with direct cooling with circulating cooling (quenching) liquids, but without great success.
The separation of the hydrogen from the hydrocarbons of the effluent must be performed at a pressure higher than that prevailing in the reaction zone. Moreover, to condense the hydrocarbons in the gas mixture constituting the effluent and containing hydrogen, it is necessary to cool to a temperature lower than that which conventional air or water heat exchangers can achieve.
Since a cooling below 0.degree. C. is required for the separation of the hydrogen and the hydrocarbon compounds, the water present in the compressed effluent must be removed down to a concentration limit such that there is no freezing, to prevent the obstruction of the cooling equipment. For this purpose, 3 .ANG. molecular sieves are used to displace the water of the effluent before the cooling step (1 .ANG.=10.sup.-10 m).
However, the adsorption beds with molecular sieves also impose pressure drops on the compression effluent due not only to the adsorption beds themselves but to systems of filters downstream from these beds which pick up the dust from molecular sieves that can obstruct the passage of the effluent in the cooling equipment. Furthermore, the molecular sieves involve periods of adsorption of the water followed by periods of regeneration which are difficult to control.